Steels for cold forging and process for producing the same

ABSTRACT

This invention provides a steel for cold forging, excellent in surface layer hardness and softening properties by annealing, which contains, in terms of wt %, C: 0.1 to 1.0%, Si: 0.1 to 2.0%, Mn: 0.01 to 1.50%, P: not greater than 0.100%, S: not greater than 0.500%, sol. N: not greater than 0.005% and the balance consisting of Fe and unavoidable impurities, wherein a pearlite ratio in the steel structure is not greater than 120×(C %) % and the outermost surface layer hardness is at least 450×(C %)+90 in terms of the Vickers hardness HV, and a production method thereof. The invention provides also a steel for cold forging, which has a structure wherein a ratio of graphite amount to the carbon content in the steel exceeds 20%, a mean grain diameter of graphite is not greater than 10×(C %) ⅓  μm and a maximum grain diameter is not greater than 20 μm.

TECHNICAL FIELD

This invention relates to a structural steel that is subjected to coldforging, either as-rolled or after rolling and annealing, and a methodof producing such a steel.

BACKGROUND ART

Steels used for structural members are passed through various formingprocesses in order to impart required properties to them.Radio-frequency hardening, for hardening the surface layer, is one ofthese processes. Since such structural members are required to have onlya high surface layer hardness, in most cases, an increase in the numberof processes results in an increase of the cost of production, and thishas been one of the problems in the past. Since as-rolled materials ofthe conventional structural steels have a low cooling rate, they have aferrite-pearlite structure in most cases. However, their surface layerhardness is low and never reaches the level achievable byradio-frequency hardening. More often than not, the surface layerhardness is lower than the internal hardness due to the influence ofdecarburization, and so forth. Though ordinary members need not alwayshave a maximum hardness corresponding to the C (carbon) content broughtforth by radio-frequency hardening, it is undeniable that some of themembers are required to have a hardness higher than that of the annealedmaterials. Therefore, the provision of steels having, as-rolled, ahigher surface layer hardness than the internal hardness has beenanother problem.

When complicated shapes are required, the steel materials are passedthrough forging and cutting processes. Because hot forging needs heatingand has a low forming accuracy, cold forging, having higher formingaccuracy, has been preferred. Nonetheless, conventional as-rolledmaterials are not suitable for cold forging because the hardness is toohigh. Ordinary steels for cold forging are generally softened byspheroidizing cementite. The annealing time is extremely long and is asmuch as about 20 hours.

The prior art references such as Japanese Unexamined Patent Publication(Kokai) No. 3-140411 describe that cold formability and cuttability ofeven a steel having a carbon content equivalent to the level of carbonsteels for cold forging can be improved by graphitizing carbon andconverting the steel structure to a ferrite-graphite dual phase.However, annealing for a long time is necessary to achieve such astructure, and the problems of production efficiency and production costare left unsolved. In other words, the problem of shortening theannealing time is yet to be solved.

In order to reduce the graphitization annealing time, a technique hasbeen suggested which adds B and uses BN as precipitation nuclei.However, when such a specific precipitate is used, atemperature-retaining process, in the BN precipitation temperaturerange, is necessary before annealing is conducted, and an additionalannealing process becomes necessary. If this heat-treatment is conductedconjointly by rolling or hot forging, temperature control must beconducted extremely strictly until annealing, and this is virtuallyimpossible.

In other words, the precipitation temperature of BN is believed to befrom about 850 to about 900° C., but rolling and hot forging areactually carried out at a temperature higher than 1,000° C. in manycases. Therefore, in order to use such a graphite-containing steel forcold forging, rolling and hot forging, as prior processes, must beconducted at a temperature below 1,000° C. Hot forming at such atemperature lowers the service life of tools such as rolls and punches.The increase of the number of limitations on the processes leads to thedrop of production efficiency, and must be therefore avoided to restrictthe increase of the production cost. From the aspects of steel makingand hot forging, as a prior process to cold forging, steel materialsthat do not need strict temperature control and can be annealed andsoftened within a short time have been required.

Japanese Unexamined Patent Publication (Kokai) No. 2-111842 teachesshortening the annealing time by restricting the graphite content withina short time. However, this technology does not provide a fundamentalsolution because cold forgeability and cuttability are deteriorated inproportion to the amount of cementite that remains in the steelmaterials as a result of suppression of the graphite content.

As described above, the conventional as-rolled materials are notentirely satisfactory because their surface layer hardness is notsufficient when they are used as such, but it is too high when they aresubjected to cold forging and cutting. From the viewpoint of production,on the other hand, there is the fundamental problem that the steelsshould preferably be produced collectively by reducing the number oftheir kinds in order to reduce the cost of production. Therefore, it hasbeen desired that the as-rolled materials have a sufficient surfacehardness, the annealing time can be shortened when the as-rolledmaterials are subjected to cold forging, and they can exhibit excellentcold forgeability after annealing.

When strength is also further required, it may be possible, inprinciple, to add those elements which do not impede graphitization forimproving hardenability but can improve hardenability. Particularly whenthe surface hardness by radio-frequency hardening is necessary,hardenability becomes more different problem because of increase thethickness of the hardened layer. However, since ordinary hardenabilityimproving elements such as Cr, Mn, Mo, etc, hinder graphitization, theamounts of addition are limited. When the graphitization annealing timeis shortened by forming BN, B cannot be used as the hardenabilityimproving element, and the hardening depth cannot be sufficientlysecured, either.

Under the above-described condition, a steel which makes it possible toreduce the annealing time, and is excellent in cold forgeability afterannealing, hardenability and cuttability, has been required.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a steel that has,as-rolled, excellent surface hardness, by regulating the chemicalcomponents of the steel and its microstructure, and can impart excellentcold forgeability within an extremely short softening/annealing timebefore cold forging and cutting, and to provide a method of producingthe steel.

It is another object of the present invention to provide a steel, forcold forging after annealing, that can shorten the annealing time, byregulating the chemical components of the steel, is excellent in coldformability and cuttability after annealing and has excellent strengthand toughness after hardening and tempering.

To accomplish these objects, the present invention provides thefollowing inventions.

(1) The first invention provides a steel for cold forging, excellent insurface layer hardness and softening properties by annealing, thatcontains, in terms of wt %, C: 0.1 to 1.0%, Si: 0.1 to 2.0%, Mn: 0.01 to1.50%, P: not greater than 0.100%, S: not greater than 0.500%, sol. N:being limited to not greater than 0.005%, and the balance consisting ofFe and unavoidable impurities, wherein a pearlite ratio in the steelstructure (pearlite occupying area ratio in microscope plate/microscopeplate area) is not greater than 120×(C %) % (with the maximum being notgreater than 100%), and the outermost surface layer hardness is at least450×(C %)+90 in terms of the Vickers hardness HV.

(2) The second invention provides a steel for cold forging, excellent insurface layer hardness and softening properties by annealing, whichcontains at least one of Cr: 0.01 to 0.70% and Mo: 0.05 to 0.50%, inaddition to the chemical components of the first invention (1) describedabove, wherein a pearlite ratio in the steel structure (pearliteoccupying area ratio in microscope plate/microscope plate area) is notgreater than 120×(C %) %, and the outermost surface layer hardness is atleast 450×(C %)+90 in terms of the Vickers hardness HV.

(3) The third invention provides a steel for cold forging, excellent insurface layer hardness and softening properties by annealing, whichcontains at least one of Ti: 0.01 to 0.20%, V: 0.05 to 0.50%, Nb: 0.01to 0.10%, Zr: 0.01 to 0.30% and Al: 0.001 to 0.050% in addition to thechemical components of the paragraph (1) or (2) described above, whereina pearlite ratio in the steel structure (pearlite occupying area ratioon microscope plate/microscope plate area) is not grater than 120×(C %)%, and the outermost surface layer hardness is at least 450×(C %)+90 interms of the Vickers hardness Hv.

(4) The fourth invention provides a steel for cold forging, excellent insurface layer hardness and softening properties by annealing, whichcontains B: 0.0001 to 0.0060% in addition to the chemical components ofany of the paragraphs (1) to (3), wherein a pearlite ratio in the steelstructure (pearlite occupying area ratio on microscope plate/microscopeplate area) is not greater than 120×(C %) %, and the outermost layersurface hardness is at least 450×(C%)+90 in terms of the Vickershardness Hv.

(5) The fifth invention provides a steel for cold forging, excellent insurface layer hardness and softening properties by annealing, whichcontains Pb: 0.01 to 0.30%, Ca: 0.0001 to 0.0020%, Te: 0.001 to 0.100%,Se: 0.01 to 0.50% and Bi: 0.01 to 0.50% in addition to the chemicalcomponents of any of the paragraphs (1) to (4), wherein a pearlite ratioin the steel structure (pearlite occupying area ratio in microscopeplate/microscope plate area) is not greater than 120×(C %) %, and theoutermost layer hardness is at least 450×(C %)+90 in terms of theVickers hardness Hv.

(6) The sixth invention provides a steel for cold forging, excellent insurface layer hardness and softening properties by annealing, whichcontains Mg: 0.0005 to 0.0200% in addition to said chemical componentsaccording to any of claims 1 through 6, wherein a pearlite ratio in thesteel structure (pearlite occupying area ratio on microscopeplate/microscope plate area) is not greater than 120×(C %) %, and theoutermost surface layer hardness is at least 450×(C %)+90 in terms ofthe Vickers hardness HV.

(7) The seventh invention provides a steel for cold forging, excellentin cold formability, cuttability and radio-frequency hardenability,which contains, in terms of wt %, C: 0.1 to 1.0%, Si: 0.1 to 2.0%, Mn:0.01 to 1.50%, P: not greater than 0.100%, S: not greater than 0.500,sol. N: being limited to not greater than 0.005% and the balanceconsisting of Fe and unavoidable impurities, and has a structure whereina ratio of graphite amount to the carbon content in the steel(graphitization ratio: amount of carbon precipitated as graphite/carboncontent in the steel) exceeds 20%, a mean crystal grain diameter of thegraphite is not greater than 10×(C %)^(⅓) μm and the maximum crystalgrain diameter is not greater than 20 μm.

(8) The eighth invention provides a steel for cold forging, excellent incold formability, cuttability and radio-frequency hardenability, whichcontains at least one of Cr: 0.01 to 0.70% and Mo: 0.05 to 0.50%, andhas a structure wherein a ratio of graphite amount to the carbon contentin the steel (graphitization ratio: amount of carbon precipitated asgraphite/carbon content in the steel) exceeds 20%, a mean crystal graindiameter of the graphite is not greater than 10×(C %)^(⅓) μm , and amaximum crystal grain diameter is not greater than 20 μm.

(9) The ninth invention provides a steel for cold forging, excellent incold formability, cuttability and radio-frequency hardenability, whichcontains at least one of Ti: 0.01 to 0.20%, V: 0.05 to 0.50%, Nb: 0.01to 0.10%, Zr: 0.01 to 0.30% and Al: 0.001 to 0.050% in addition to thechemical components described in the paragraph (7) or (8), and has astructure wherein a ratio of graphite amount to the carbon content inthe steel (graphitization ratio: amount of carbon precipitated asgraphite/carbon content in the steel) exceeds 20%, a mean crystal graindiameter of the graphite is not greater than 10×(C %)^(⅓) μm, and amaximum crystal grain diameter is not greater than 20 μm.

(10) The tenth invention provides a steel for cold forging, whichcontains B: 0.0001 to 0.0060% in addition to the chemical components ofany of the paragraphs (7) to (9), and has a structure wherein a ratio ofgraphite amount to the carbon content in the steel (graphitizationratio: amount of carbon precipitated as graphite/carbon content in thesteel) exceeds 20%, a mean crystal grain diameter of the graphite is notgreater than 10×(C %)^(⅓) μm and a maximum crystal grain diameter is notgreater than 20 μm.

(11) The eleventh invention provides a steel for cold forging, excellentin cold formability, cuttability and radio-frequency hardenability,which contains Pb: 0.01 to 0.30%, Ca: 0.0001 to 0.0020%, Te: 0.001 to0.100%, Se: 0.01 to 0.50% and Bi: 0.01 to 0.50% in addition to thechemical components of any of the paragraphs (7) to (10), and has astructure wherein a ratio of a graphite amount to the carbon content inthe steel (graphitization ratio: amount of carbon precipitated asgraphite/carbon content in the steel) exceeds 20%, a mean crystal graindiameter of graphite is not greater than 10×(C %)^(⅓) μm, and a maximumcrystal grain diameter is not greater than 20 μm.

(12) The twelfth invention provides a steel for cold forging, excellentin cold formability, cuttability and radio-frequency hardenability,which contains Mg: 0.0005 to 0.0200% in addition to the chemicalcomponents of any of the paragraphs (7) to (11), and has a structurewherein a ratio of graphite amount to the carbon content in the steel(graphitization ratio: amount of carbon precipitated as graphite/carboncontent in the steel) exceeds 20%, a mean crystal grain diameter of thegraphite is not greater than 10×(C %)^(⅓) μm, and a maximum crystalgrain diameter is not greater than 20 μm.

(13) A method of producing a steel for cold forging, excellent insurface layer hardness and softening properties by annealing, whichcomprises the steps of rolling the steel having the chemical componentsof any of the paragraphs (1) to (6) described above in an austenitetemperature zone or in an austenite-ferrite dual phase zone so that apearlite ratio in the steel structure (pearlite occupying area ratio inmicroscope plate/microscope plate area) is not greater than 120×(C %) %and the outermost surface layer hardness is at least 450×(C %)+90 interms of the Vickers hardness Hv; rapidly cooling the steel immediatelyafter the finish of rolling at a rate of at least 1° C./s; andcontrolling a recuperative temperature to 650° C. or below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing the outline of a pearlite ratiomeasuring method.

FIG. 2 is a graph showing the relation between a pearlite area ratio andan annealing time until softening in an embodiment of a 0.20% class.

FIG. 3 is a graph showing the relation between the pearlite area ratioand the annealing time until softening in an embodiment of a 0.35%class.

FIG. 4 is a graph showing the relation between the pearlite area ratioand the annealing time until softening in an embodiment of a 0.45%class.

FIG. 5 is a graph showing the relation between the pearlite area ratioand the annealing time until softening in an embodiment of 0.55% class.

FIG. 6 is a graph showing the relation between a recuperativetemperature and a surface layer hardness.

FIG. 7 is a graph showing the relation between the recuperativetemperature and the pearlite area ratio.

FIG. 8 is a graph showing the relation between solid solution nitrogenand the annealing time until softening.

FIG. 9 is a graph showing the relation between a maximum crystal graindiameter and a hardening time by radio-frequency heating in anembodiment of a 0.55% C class.

FIG. 10 is a graph showing the relation between a mean crystal graindiameter and the hardening time by radio-frequency heating in anembodiment of the 0.55 C class.

FIG. 11 is a graph showing the relation between the mean crystal graindiameter and the hardening time by radio-frequency heating in anembodiment of the 0.35% C class.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be explained in detail.

Initially, the steel structure used for the steel for cold forgingaccording to the present invention, and its contents, will be explained.

At least 0.1% of C (carbon) must be contained in order to securestrength as components after hardening and tempering. The upper limit isset to 1.0% to prevent firing cracking.

Si (silicon) has the function of promoting graphitization by increasingcarbon activity in the steel. Its lower limit is preferably at least0.1% from the aspect of graphitization. If the Si content exceeds 2.0%,problems such as the increase of ferrite hardness and the loss oftoughness of the steel become remarkable. Therefore, the upper limit is2.0%. Si can be used as the element that regulates the graphitizationratio. The smaller its content, the smaller becomes the graphitizationratio after annealing. When the graphitization ratio is lowered bydecreasing the Si content, the hardness of the ferrite phase drops.Therefore, the hardness of the steel material does not increase withinthe range described above, and cold forgeability is not lowered.

Mn (manganese) must be added in the total amount of the amount requiredfor fixing and dispersing S in the steel as MnS and the amount requiredfor securing the strength after hardening by causing Mn to undergo solidsolution in the matrix. Its lower limit value is 0.01%. The hardness ofthe base becomes higher with the increase of the Mn content, and coldformability drops. Mn is also a graphitization-impeding element. Whenthe amount of addition increases, the annealing time is likely to becomelonger. Therefore, the upper limit is set to 1.50%.

P (phosphorus) increases the hardness of the base metal in the steel andlowers cold formability. Therefore, its upper limit must be 0.1000%.

S (sulfur) exists as MnS inclusions as it combines with Mn. From theaspect of cold formability, its upper limit must be set to 0.500%.

Solid solution nitrogen, that does not exist as nitrides, dissolves incementite and impedes decomposition of cementite. Therefore, it is agraphitization-impeding element. Therefore, the present inventionstipulates N as sol. N. If the sol. N content exceeds 0.005%, theannealing time necessary for graphitization becomes extremely long.Therefore, the upper limit of sol. N is 0.005%. This is because sol. Nhinders the diffusion of C, retards graphitization and enhances theferrite hardness.

Cr (chromium) is a hardenability-improving element and at the same time,a graphitization-impeding element. Therefore, when the improvement ofhardenability is required, at least 0.01% of Cr must be added. Whenadded in a large amount, Cr impedes graphitization and prolongs theannealing time. Therefore, the upper limit is 0.70%.

Mo (molybdenum) is the element that increases the strength afterhardening, but is likely to form carbides and impedes graphitization.Therefore, the upper limit is set to 0.50% at which thegraphitization-impeding effect becomes remarkable, and the Mo content isset to the addition amount that does not greatly impede the formation ofthe graphite nuclei. In comparison with other hardenability-improvingelements, however, the degree of impeding of graphitization by Mo issmaller. For this reason, the Mo addition amount may be increased so asto improve hardenability within the range stipulated above.

Ti (titanium) forms TiN in the steel and reduces the γ grain diameter.Graphite is likely to precipitate at the γ grain boundary andprecipitates, or in other words, “non-uniform portions” of the lattice,and carbonitrides of Ti bear the role of the precipitation nuclei ofgraphite and the role of creation of the graphite precipitation nucleidue to the reduction of the γ grain diameters to fine diameters.Furthermore, Ti fixes N as the nitrides and thus reduces sol. N. If theTi content is less than 0.01%, its effect is small, and if the Ticontent exceeds 0.20%, the effect gets into saturation and at the sametime, a large amount of TiN is precipitated and spoil the mechanicalproperties.

V (vanadium) forms carbonitrides, and shortens the graphitizationannealing time from both the aspect of fining of the γ grains and of theprecipitation nuclei. It reduces sol. N at the time of the formation ofcarbonitrides. If the V content is less than 0.05%, its effect is small,and if the V content exceeds 0.50%, the effect gets into saturation andat the same time, large amounts of non-dissolved carbides remain withthe result being deterioration of the mechanical properties.

Nb (niobium) forms carbonitrides and shortens the graphitizationannealing time from both the aspect of fining of the γ grain diametersto fine diameters and of the precipitation nuclei. It also lowers sol. Nat the time of the formation of the nitrides. If the Nb content is lessthan 0.01%, the effect is small and if it exceeds 0.10%, the effect getsinto saturation and at the same time, large amounts of non-dissolvedcarbides remain with the result being deterioration of the mechanicalproperties.

Mo (molybdenum) increases the strength after hardening. However, it isthe element that is likely to form carbides, lowers carbon activity, andimpedes graphitization. Therefore, the upper limit is set to 0.5% atwhich the graphitization-impeding effect becomes remarkable, and theaddition amount is limited to the level at which the graphite nucleusformation is not greatly impeded. Since the degree of thegraphitization-impeding effect of Mo is lower than that of otherhardenability-improving elements, however, the Mo addition amount may beincreased so as to improve hardenability within the range stipulatedabove.

Zr (zirconium) forms oxides, nitrides, carbides and sulfides, whichshorten the graphitization annealing time as the precipitation nuclei.Zr reduces sol. N at the time of the formation of the nitrides.Furthermore, Zr spheroidizes the shapes of the sulfides such as MnS, andcan mitigate rolling anisotropy as one of the mechanical properties.Furthermore, Zr can improve hardenability. If the Zr content is lessthan 0.01%, the effect is small and if it exceeds 0.30%, the effect getsinto saturation and at the same time, large amounts of non-dissolvedcarbides remain with the result being deterioration of the mechanicalproperties.

At least 0.001% of Al (aluminum) is necessary for deoxidizing the steeland for preventing surface scratches during rolling. The deoxidizingeffect gets into saturation when the Al content exceeds 0.050% and theamounts of aluminum type inclusions increase. Therefore, the upper limitis 0.050%. When precipitated as AlN, aluminum plays the role of theprecipitation nuclei of graphite and the role of creating the graphiteprecipitation nuclei due to fining of the y grain diameters to finediameters. Furthermore, because Al fixes N as the nitrides, it reducessol. N.

B (boron) reacts with N and precipitates as BN in the austenite crystalgrain boundary. It is therefore useful for reducing sol. N. BN has ahexagonal system as its crystal structure in the same way as graphite,and functions as the precipitation nuclei of graphite. Furthermore, sol.B is the element that improves hardenability, and is preferably addedwhen hardenability is required. Its lower limit value must be 0.0001%.The effects of precipitating BN and improving hardenability get intosaturation when the B content exceeds 0.0060%. Therefore, the upperlimit is 0.0060%.

Pb (lead) is a cuttability-improving element, and at least 0.01% isnecessary when cuttability is required. If the Pb content exceeds 0.30%,Pb impedes graphitization and invites problems during production such asrolling scratches. Therefore, the upper limit is 0.30%.

Ca (calcium) is effective when mitigation of rolling anisotropy byspheroidizing of MnS and the improvement of cuttability are required. Ifthe Ca content is less than 0.0001%, the effect is small, and if itexceeds 0.0020%, the precipitates will deteriorate the mechanicalproperties. Therefore, the upper limit is 0.0020%.

Te (tellurium) is a cuttability-improving element and helps mitigaterolling anisotropy by spheroidizing of MnS. If the Te content is lessthan 0.001%, the effect is small and if it exceeds 0.100%, problems suchas impediment of graphitizing and rolling scratches occur. Therefore,the upper limit is 0.100%.

Se (selenium) is effective for improving cuttability. If the Se contentis less than 0.01%, the effect is small, and if it exceeds 0.50%, theeffect gets into saturation. Therefore, the upper limit is 0.50%.

Bi (bismuth) is effective for improving cuttability. If the Bi contentis less than 0.01%, the effect is small, and if it exceeds 0.50%, theeffect gets into saturation. Therefore, the upper limit is 0.50%.

Mg (manganese) is an element that forms oxides such as MgO and alsoforms sulfides. MgS is co-present with MnS in many cases and such oxidesand sulfides function as the graphite precipitation nuclei and areeffective for finely dispersing graphite and for shortening theannealing time. If the Mg content is less than 0.0005%, the effectcannot be observed and if it exceeds 0.0200%, Mg forms large amounts ofoxides and lowers the strength of the steel. Therefore, the Mg contentis limited to the range of 0.0005 to 0.0200%.

Next, the as-rolled steel structure of the steel for cold forgingaccording to the present invention will be explained.

The hardness of the surface layer of the steel for cold forging can beincreased by rapidly cooling the steel from a temperature above atransformation point, but is affected by the C content. When the surfacelayer hardness is too low, the steel cannot be used for the applicationthat requires the surface layer hardness. For example, those steels forwhich wear resistance is required must have hardness at least higherthan the strength of ordinary annealed steel materials. The presentinvention can provide a steel having hardness of at least 450×(C %)+90in terms of the Vickers hardness Hv in accordance with the C content.

Next, the reason why the pearlite ratio in the steel structure, that is,(pearlite occupying area ratio in microscope plate/microscope plate), islimited to not greater than 120×(C %) % (with the proviso that the valueis not greater than 100%; and hereinafter the same) will be explained.When carbon in the steel is graphitized in the component system of thepresent invention, cementite is generally formed if the steel is cooledfrom the austenite region at an atmospheric cooling rate or a ratehigher than the former. In order to impart excellent cold formabilityafter annealing, however, carbon (C) must be graphitized by annealing.The graphitization process by annealing is believed to comprisedecomposition of cementite→diffusion of C→formation and growth ofgraphite nuclei. From the viewpoint of the decomposition of cementite, along time is necessary for the decomposition of cementite if the size ofcementite is great and it is stable energy-wise, that is, if C formspearlite on the lamella. In consequence, the annealing time cannot beshortened.

From the viewpoint of the growth of graphite, graphite at positionshaving a small diffusion distance for C are likely to be formed and togrow. In other words, graphite is likely to be formed near the positionsof previous pearlite. This means that the graphite so formed is coarseand is non-uniformly dispersed. The deformation quantity till breakageafter annealing is decreased, decomposition of graphite byradio-frequency hardening and diffusion of C are time-consuming, andhardening properties by radio-frequency hardening are lowered. In thisway, in the steel according to the present invention, the formation ofpearlite is restricted as much as possible so that the annealing timecan be shortened and excellent deformation properties can be impartedafter annealing.

Next, the outline of the method of measuring the pearlite ratio is shownin FIG. 1. The calculation method of the pearlite ratio by the pearliteratio measuring method is made in accordance with the followingequation.$( {P\quad \%} ) = {\sum\limits_{i = 1}^{n}\{ {{( {{Pi}\quad \%} ) \cdot 2}\pi \quad {w \cdot {ri}}} \}}$

Here,

ri=(i−1)·w+w/2, w =R/n

(P%)=pearlite ratio,

w: measurement representative width,

n: number of splitting

(Pi%): pearlite proportion at measurement position,

ri: measurement representative radius,

i: argument at the time of splitting (I=1, 2, . . . , n) from inside),

R: radius of steel bar or wire material.

This method is a simple method. The greater the number of splitting n,the smaller becomes w. Therefore, the pearlite ratio of the steel can becalculated as a correct area ratio.

The present invention stipulates n to ≧5. More concretely, a polishedsample for microscope inspection, which is etched in a sectionaldirection by a nital reagent, is inspected in a 1 mm pitch from thesurface layer to the center through a 1,000× optical microscope (n=10 ina 20 mm wire material). The pearlite area ratio inside the visual fieldis measured by an image processor, and the pearlite area occupying ratioinside the section is calculated using the area ratio as arepresentative value w of a 1 mm width in the radial direction of thesteel bar or the wire material.

In this case, the samples in which the lamella structure can be observedby etching by the nital reagent are defined as pearlite. When this arearatio exceeds 120×(C %) %, the annealing time is extremely extended. Theinfluences on the annealing time vary with the C content of the rawmaterial. However, if the C content is great and the pearlite areaoccupying ratio is greater than 120×(C %) %, the material cannot bepractically used from the aspect of the production cost. Therefore, theupper limit of the pearlite area ratio is limited to 120×(C %) %.However, this value does not exceed 100%.

FIGS. 2 to 5 show the relation between the pearlite area ratio beforeannealing and the annealing time when the C content is different,respectively. The steel is softened more easily when the C content issmaller, but the annealing time is extremely prolonged outside the rangeof the present invention, as can be seen from these graphs.

Next, the steel structure of the steel for cold forging according to thepresent invention, after it is hardened or annealed, will be explained.

The majority of C in the steel exists as cementite or graphite. Graphitecan easily undergo deformation because it has cleavages. If the matrixis soft, cold forgeability is excellent. When the steel is cut,cuttability can be improved by the functions of both an internallubricant and a breaking starting point. If the graphite content issmaller than 20%, the steel cannot exhibit sufficientdeformation/lubricating functions. Therefore, the graphite content mustexceed 20%. When deformation properties are preferentially required, thegraphitization is increased. In order to secure excellentradio-frequency hardenability, on the other hand, it is effective tointentionally leave a part of C without being graphitized and to leaveit as cementite.

Furthermore, the present invention stipulates that the mean crystalgrain diameter of graphite is not greater than 10×(C%)^(⅓) μm and themaximum grain diameter is not greater than 20 μm, in consideration ofradio-frequency hardenability. In other words, when radio-frequencyhardening is conducted, the hardening properties are governed bydecomposition/diffusion of C in graphite. In this instance, if thegraphite grain diameter is great, a large quantity of energy and muchtime are necessary for the decomposition/diffusion, and a stablehardened layer cannot be obtained easily by radio-frequency hardening.In order to stably obtain the hardened layer corresponding to the Ccontent contained in the steel by radio-frequency hardening, the processof which can be finished within a short time, the mean grain diameter ofgraphite must be not greater than 10×(C %)^(⅓) μm. If the mean graindiameter exceeds this limit, the amount of non-dissolved graphite isgreat even after radio-frequency hardening, or the amount of a mixedstructure of a layer containing C in the diffusion process and ferritethat does not yet contain diffused C becomes great. As a result, notonly hardening becomes difficult, but a stabilized hardened layer cannotbe obtained.

FIGS. 10 and 11 show the relation between the mean grain diameter ofgraphite and the hardening time by radio-frequency hardening, and FIG. 9shows the relation between the maximum grain diameter of graphite andthe hardening time by radio-frequency hardening.

Next, the production method when the steel for cold forging according tothe present invention is used as-rolled will be explained.

After the steel having the steel composition described above is rolledin the austenite temperature range, the formation quantity of pearlitewill become great if the cooling rate is low, and the annealing timetill softening gets prolonged. Because the surface layer hardness is notsufficient, either, the steel is so soft that it cannot be used directlyas such and is too hard for cold forging. To solve these problems, thesteel is preferably cooled rapidly. If the cooling rate of the surfacelayer from the end of rolling to 500° C. is at least 1° C./s, thehardness at the surface layer can be increased in comparison with thehardness of the inside that is gradually cooled. In order to keep thepearlite area ratio on the steel section at 120×(C %) % or below, too,cooling must be carried out at a cooling rate of at least 1° C./s. Theaustenite amount can be decreased by once cooling the steel, heating itagain to the austenitization temperature, and then cooling it by water.However, on-line treatment is more preferred from the aspects of theproduction cost and the production process.

In connection with the internal structure of the steel, the main objectof the present invention is not to increase the hardness by rapidcooling as in the case of ordinary hardening but is to prevent theformation of pearlite so that decomposition easily develops duringannealing. For this reason, the cooling capacity need not particularlybe increased. In the practical production process of the steelmaterials, products having diameters of 5 to 150 mm are shipped in mostcases, and the present invention may be directed to restrict theformation of pearlite in such products. In other words, the steelstructure need not particularly comprise the martensite structure, andeven the structure having the bainite structure can shorten theannealing time for softening much more than the steels having theferrite and pearlite structures. Concrete means pass the steel materialimmediately after rolling through a cooling apparatus such as a coolingtrough or a water tank that is installed at the rearmost part of therolling line.

In the on-line process, the steel material is passed through the coolingmeans and is then cooled in the open atmosphere. It is hereby importantthat even when the surface layer is once cooled, it is heatedrecuperatively by the heat inside the steel material. It is necessary tolimit this recuperative temperature to 650° C. or below.

If the recuperative temperature is higher than 650° C., the surfacelayer hardness drops, and pearlite is formed at a part of the structureduring cooling of the steel material in the open atmosphere. Therefore,it becomes difficult to limit the pearlite amount to 120×(C %) %. Thecooling rate and the recuperative properties are greatly affected by thediameters of the rods and the wires that are rolled. Cooling means isnot limited to water cooling, and any means capable of achieving thecooling rate of at least 1° C./sec and the recuperative temperature ofnot higher than 650° C. may be employed, such as oil cooling, aircooling, and so forth.

As described above, the steel material is cooled immediately afterrolling by the cooling means mounted to the rolling line, and therecuperative temperature is limited to 650° C. or below. In this way,the surface layer hardness can be increased and the pearlite areaoccupying ratio can be limited to 120×(C %) % or below.

FIG. 6 shows the relation between the recuperative temperature and thesurface layer hardness. As shown in FIG. 6, the surface layer hardnesscannot be secured when the recuperative heat becomes high. FIG. 7 showsthe relation between the recuperative temperature and the pearlite arearatio. It can be seen from FIG. 7 that the pearlite area ratio increaseswhen the recuperative temperature becomes high. It can be thusappreciated from FIGS. 6 and 7 that restriction of the recuperativetemperature after rapid cooling is of importance.

Next, the annealing condition when the steel for cold forging, that isproduced in accordance with the present invention and is used for coldforming after annealing, will be explained.

In order to obtain graphite in the amount stipulated by the presentinvention for using the steel for cold forming, annealing is furthernecessary. Since graphite is a stable phase of the steels in Fe—C typesteels, the steels may be kept at a temperature lower than thetransformation temperature A, for a long time. However, since it ispractically necessary to precipitate graphite within a limited time, thesteels are preferably kept at a temperature within the range of 600 to710° C. at which graphite precipitates more quickly. In this case,graphitization can be completed within 1 to 50 hours.

When such a condition is employed, the structure, in which the existenceratio of C as graphite in the steel exceeds 20%, the mean grain diameterof graphite is not greater than 10×(C %)^(⅓) μm and the maximum graindiameter is not greater than 20 μm, as stipulated in the presentinvention, can be acquired.

EXAMPLES Example 1

Steels having the chemical components shown in Tables 1 to 8 weremelted. In this example, the steels were rolled into a diameter of 50 mmor 20 mm in the austenite temperature zone and were immediately cooledwith water. The rolling temperatures were within the range of 800 to1,100° C. falling within the austenite temperature zone. Water coolingwas conducted using a cooling trough installed at the rearmost part ofthe rolling line. Some of test specimens inclusive of ComparativeExamples were rolled to a diameter of 500 mm or 20 mm at temperatureshigher than 1,200° C. and were then cooled by air.

A specimen for optical microscope study was collected from each teststeel in the sectional direction and, after being polished into a mirrorsurface, each specimen was etched using nital. Pearlite was isolatedfrom other structures at a magnification of 1,000×, and the pearlitearea ratio was quantitatively determined by an image processor. In thiscase, the number of visual fields, as the object, was 50.

Such heat-treated materials were annealed at 680° C. To determine thehardness, the hardness was measured every four hours up to the annealingtime of 16 hours, every 8 hours up to the annealing time of 48 hours andevery 24 hours after the annealing time of longer than 48 hours. TheVickers hardness was determined by the annealing time at which thehardness dropped below HV: 130. As to the temperature, the surfacetemperatures of the steel materials were measured by a radiationpyrometer. The cooling rate was obtained by dividing the temperaturedifference between the temperature immediately before cooling and thetemperature after recuperation, by the time required for recuperation.

Tables 1 to 6 illustrate examples of the present invention (Nos. 1 to42) and Tables 7 and 8 show Comparative Examples (Nos. 43 to 62). All ofthe examples of the present invention had a high surface hardness, andthe softening annealing time was short, too. In Comparative Examples 43to 54, however, the annealing time for softening was prolonged when thesol. N amount was outside the range of the present invention. InComparative Examples 55 to 59, the pearlite fraction was great becausethe cooling rate was insufficient, and the annealing time was long. InComparative Examples 60 to 62, the recuperative temperature was high andthe annealing time was long, too. It could be appreciated that thesurface layer hardness was insufficient when the cooling rate and therecuperative temperature were outside the respective ranges stipulatedby the present invention.

TABLE 1 chemical components No. section C Si Mn P S sol. N Cr Ti V Nb ZrMo 1 Example of 0.51 1.23 0.32 0.023 0.017 0.0020 this invention 2Example of 0.54 1.87 0.82 0.023 0.017 0.0021 this invention 3 Example of0.56 1.43 1.21 0.008 0.008 0.0019 0.021 this invention 4 Example of 0.521.17 0.45 0.012 0.030 0.0042 0.20 this invention 5 Example of 0.51 1.230.32 0.023 0.017 0.0020 0.11 this invention 6 Example of 0.54 1.87 0.820.023 0.017 0.0021 0.022 this invention 7 Example of 0.56 1.43 1.210.008 0.008 0.0019 0.023 this invention 8 Example of 0.52 1.17 0.450.012 0.030 0.0042 0.035 this invention 9 Example of 0.51 1.16 0.450.027 0.028 0.0035 0.12 this invention 10 Example of 0.48 1.26 0.280.024 0.021 0.0019 0.11 this invention 11 Example of 0.54 1.82 0.540.024 0.021 0.0029 0.05 this invention 12 Example of 0.48 1.26 0.360.029 0.018 0.0037 this invention 13 Example of 0.51 1.29 0.38 0.0210.015 0.0032 this invention 14 Example of 0.53 1.25 0.36 0.029 0.0180.0037 this invention

TABLE 2 anneal- Cool- recuper- pear- anneal- ing ing ative surface liteing hard- chemical components rate temp. layer ratio time ness No.section Al B Pb Ca Te Se Bi Mg (° C./s) (° C.) (HV) (%) (hr) (HV) 1Example of 0.027 15 100 652 0 8 121 this invention 2 Example of 0.023 8489 429 0 8 124 this invention 3 Example of 0.017 3 520 364 25 16 127this invention 4 Example of 10 560 410 23 16 126 this invention 5Example of 0.027 3 510 319 12 8 121 this invention 6 Example of 0.023 15380 621 0 8 124 this invention 7 Example of 0.017 8 490 510 11 8 127this invention 8 Example of 8 420 565 7 8 126 this invention 9 Exampleof 0.022 8 380 589 0 8 119 this invention 10 Example of 0.034 0.0021 5510 405 10 16 126 this invention 11 Example of 0.029 0.13 5 410 425 3216 127 this invention 12 Exampleof 0.027 0.0021 0.0013 10 530 385 15 16124 this invention 13 Example of 0.021 0.0025 0.031 10 550 398 35 8 125this invention 14 Example of 0.023 0.0024 0.23 15 620 320 53 32 120 thisinvention

TABLE 3 chemical components No. section C Si Mn P S sol. N Cr Ti V Nb ZrMo 15 Example of 0.32 1.23 0.42 0.013 0.027 0.0021 this invention 16Example of 0.32 1.27 0.54 0.023 0.012 0.0022 this invention 17 Exampleof 0.26 1.83 0.51 0.003 0.015 0.0037 this invention 18 Example of 0.321.17 0.45 0.020 0.025 0.0012 this invention 19 Example of 0.25 1.20 0.600.026 0.020 0.0042 0.21 this invention 20 Example of 0.34 1.32 0.250.022 0.025 0.0032 0.25 0.022 this invention 21 Example of 0.35 1.210.36 0.019 0.022 0.0022 0.023 this invention 22 Example of 0.35 1.190.81 0.027 0.023 0.0038 0.035 0.25 this invention 23 Example of 0.231.16 0.52 0.028 0.023 0.0045 0.040 this invention 24 Example of 0.351.26 0.55 0.027 0.019 0.0025 0.048 this invention 25 Example of 0.311.26 0.75 0.028 0.025 0.0033 0.22 this invention 26 Example of 0.38 1.460.18 0.025 0.029 0.0015 0.10 this invention 27 Example of 0.32 1.31 0.910.030 0.022 0.0042 this invention 28 Example of 0.32 1.20 0.34 0.0210.026 0.0042 this invention 29 Example of 0.33 1.26 0.36 0.028 0.0180.0037 this invention 30 Example of 0.38 1.34 0.45 0.029 0.017 0.0026this invention

TABLE 4 anneal- Cool- recuper- pear- anneal- ing ing ative surface liteing hard- chemical components rate temp. layer ratio time ness No.section Al B Pb Ca Te Se Bi Mg (° C./s) (° C.) (HV) (%) (hr) (HV) 15Example of 0.025 15 100 652 32 4 119 this invention 16 Example of 0.0223 390 385 12 4 125 this invention 17 Example of 0.022 3 280 275 19 12124 this invention 18 Example of 15 100 398 0 4 122 this invention 19Example of 0.021 15 100 310 0 4 128 this invention 20 Example of 0.018 8330 416 0 4 124 this invention 21 Example of 0.030 3 390 402 2 4 118this invention 22 Example of 0.031 3 360 420 0 4 125 this invention 23Example of 0.029 3 480 295 5 4 126 this invention 24 Example of 0.027 3530 361 10 8 119 this inventionn 25 Example of 0.017 3 480 311 9 8 120this invention 26 Example of 0.023 0.0028 3 390 451 0 8 118 thisinvention 27 Example of 0.026 0.0025 0.021 3 470 338 17 8 131 thisinvention 28 Example of 0.022 0.0022 0.0016 3 610 306 25 16 125 thisinvention 29 Example of 0.022 0.0023 0.25 3 500 318 17 8 109 thisinvention 30 Example of 0.025 3 510 298 20 8 121 this invention

TABLE 5 chemical components No. section C Si Mn P S sol. N Cr Ti V Nb ZrMo 31 Example of 0.55 0.75 0.31 0.023 0.017 0.0020 this invention 32Example of 0.44 0.65 0.72 0.023 0.017 0.0021 this invention 33 Exampleof 0.36 0.50 1.01 0.008 0.008 0.0019 this invention 34 Example of 0.220.42 0.52 0.012 0.030 0.0042 this invention 35 Example of 0.54 0.46 0.420.021 0.019 0.0022 0.25 this invention 36 Example of 0.54 0.21 0.510.024 0.021 0.0042 0.21 0.021 this invention 37 Example of 0.55 0.550.36 0.022 0.024 0.0022 0.025 this invention 38 Example of 0.48 0.640.24 0.024 0.021 0.0048 0.025 0.21 this invention 39 Example of 0.520.43 0.37 0.022 0.022 0.0035 0.031 this invention 40 Example of 0.650.51 0.38 0.017 0.012 0.0025 0.053 this invention 41 Example of 0.510.35 0.48 0.027 0.028 0.0035 0.12 this invention 42 Example of 0.48 0.650.19 0.024 0.021 0.0019 0.11 this invention

TABLE 6 anneal- Cool- recuper- pear- anneal- ing ing ative surface liteing hard- chemical components rate temp. layer ratio time ness No.section Al B Pb Ca Te Se Bi Mg (° C./s) (° C.) (HV) (%) (hr) (HV) 31Example of 0.027 10 430 521 0 8 121 this invention 32 Example of 0.02315 100 521 0 8 124 this invention 33 Example of 0.017 3 500 320 0 4 127this invention 34 Example of 3 380 325 0 8 126 this invention 35 Exampleof 0.029 10 370 596 0 12 125 this invention 36 Example of 0.019 0.002110 440 562 36 16 122 this invention 37 Example of 0.029 8 430 545 37 12120 this invention 38 Example of 0.030 0.0021 3 550 320 42 24 128 thisinvention 39 Example of 0.036 0.0025 3 560 410 45 16 124 this invention40 Example of 0.021 0.0024 8 440 495 35 16 126 this invention 41 Exampleof 0.022 3 470 452 31 16 119 this invention 42 Example of 0.034 8 390495 2 12 126 this invention

TABLE 7 chemical components No. section C Si Mn P S sol. N Cr Ti V Nb ZrMo 43 Comparative 0.55 1.23 0.34 0.019 0.017 0.0059 Example 44Comparative 0.49 1.19 0.40 0.021 0.020 0.0070 Example 45 Comparative0.35 1.18 0.35 0.021 0.026 0.0062 Example 46 Comparative 0.53 0.75 0.410.029 0.027 0.0057 Example 47 Comparative 0.46 0.69 0.41 0.022 0.0210.0061 Example 48 Comparative 0.36 0.72 0.34 0.024 0.021 0.0057 Example49 Comparative 0.58 1.28 0.50 0.021 0.026 0.0082 0.01 Example 50Comparative 0.46 0.73 0.34 0.023 0.019 0.0059 Example 51 Comparative0.36 0.72 0.34 0.024 0.021 0.0057 Example 52 Comparative 0.58 1.21 0.320.024 0.026 0.0068 0.11 Example 53 Comparative 0.48 1.06 0.35 0.0210.022 0.0063 0.014 Example 54 Comparative 0.48 0.71 0.50 0.029 0.0210.0065 Example 55 Comparative 0.53 1.12 0.36 0.022 0.027 0.0035 Example56 Comparative 0.51 1.21 0.35 0.019 0.019 0.0038 Example 57 Comparative0.54 1.87 0.82 0.023 0.017 0.0021 Example 58 Comparative 0.46 1.43 1.210.008 0.008 0.0019 0.021 Example 59 Comparative 0.35 1.23 0.42 0.0210.016 0.0045 Example 60 Comparative 0.22 1.17 0.45 0.012 0.030 0.00420.20 Example 61 Comparative 0.51 1.23 0.32 0.023 0.017 0.0020 Example 62Comparative 0.54 1.87 0.82 0.023 0.017 0.0021 0.022 Example

TABLE 8 anneal- Cool- recuper- pear- anneal- ing ing ative surface liteing hard- chemical components rate temp. layer ratio time ness No.section Al B Pb Ca Te Se Bi Mg (° C./s) (° C.) (HV) (%) (hr) (HV) 43Example of 0.028 10 450 586 0 120 138 Example 44 Comparative 0.0190.0026 10 550 546 0 120 141 Example 45 Comparative 0.021 6 560 405 10120 145 Example 46 Comparative 0.028 8 540 486 10 120 145 Example 47Comparative 0.019 8 500 456 40 120 141 Example 48 Comparative 0.0210.0024 10 450 385 55 120 135 Example 49 Comparative 0.010 10 450 367 25120 150 Example 50 Comparative 0.019 10 570 341 20 120 141 Example 51Comparative 0.021 0.0024 10 570 345 15 120 135 Example 52 Comparative0.015 10 400 520 0 120 152 Example 53 Comparative 0.027 0.0021 10 420512 0 120 148 Example 54 Comparative 0.021 0.0021 10 440 465 16 32 148Example 55 Comparative 0.028 0.0025 0.5 770 265 86 48 125 Example 56Comparative 0.027 0.0028 0.5 700 253 90 32 126 Example 57 Comparative0.023 0.5 780 243 82 120 124 Example 58 Comparative 0.017 0.5 760 225 7570 127 Example 59 Comparative 0.024 0.5 770 205 36 48 124 Example 60Comparative 2 780 211 36 120 126 Example 61 Comparative 0.027 2 750 25492 72 151 Example 62 Comparative 0.023 2 720 259 81 96 164 Example

Example 2

Steels having the chemical components shown in Tables 9 to 16 weremelted, and were rolled into a diameter of 50 mm or 30 mm at 750 to 850°C. Some of the test specimens inclusive of Comparative Examples wereforged at a temperature above 1,200° C. Rolled materials, as examples ofthe present invention, were cooled with water by an on-line watercooling apparatus from 800 to 900° C. immediately after rolling. Theforged materials were heated to 850° C. by a heating furnace. Theexamples of the present invention were cooled by water while theComparative Examples were cooled by air or water. When air cooling wasconducted, the grain diameter of graphite became great. The size of thetest specimens in this case was 30 mm in diameter and 40 mm in length.After cooling, the heat-treated materials were heated again to 680° C.and annealed. The graphitization ratio was measured in accordance withJIS G 1211.

The polished samples were prepared, and the graphite grain diameter wasmeasured in the number of 50 visual fields and in magnification of atleast 400 times by an image processor. After graphitization annealing, ameasurement of the hardness, a cutting test and a radio-frequencyhardening test were conducted. The cutting test was carried out byboring using a high-speed steel drill having a diameter of 3 mmφ. Thistest was done while the cutting speed was changed, and the drillperipheral speed at which the tool life of at least 1,000 mm, orso-called VL 1,000 (m/min), was reached, and this value was used as theindex. This was wet cutting using a water-soluble oil at a feed quantityof 0.33 mm/rev.

The results are shown in Tables 17 to 19.

These tables show the hardness before and after annealing and thehardening time by radio-frequency hardening. The examples of the presentinvention (Nos. 1 to 59) had a hardness around HV: 120 before annealingand could be hardened to around HV: 600 after annealing. Hardenabilityby radio-frequency hardening was evaluated by a transformation pointautomatic measuring equipment (“Formaster”). When heating to 1,000° C.and rapid cooling were conducted by the Formaster, variance occurred inthe hardness after radio-frequency annealing because graphite had a slowdiffusion time. Therefore, the time before this variance of the hardnessdue to hardening disappeared was measured by changing the heating timeand conducting rapid cooling, and hardenability was evaluated by thistime. The size of each test specimen was 3 mm in diameter and 10 mm inlength. Here, the variance of hardness was regarded as havingdisappeared when the variance of hardness of five test specimens fellbelow HV: 200.

The steels of the examples of the present invention could be softenedsufficiently within the short annealing time, and had excellentmachinability. Since machinability VL1,000=150 m/min was the limit ofthe tester, the steels had the possibility of further improvement.Though soft, they were hardened without variance by radio-frequencyannealing. The annealing time was 3 seconds, and the steels could beannealed sufficiently by radio-frequency annealing without variance inthe shortest time that could be controlled by the Formaster tester.These tendencies did not change fundamentally even when elements such asTi and Cr were added, and these elements could be added whenevermachinability and hardenability were further required.

Comparative Examples Nos. 57 to 70 were test specimens the N content ofwhich exceeded the range of the present invention, and the graphitegrain diameter of which exceeded the range of the present invention. Inorder to further clarify the effect of sol. N, FIG. 8 shows theinfluences of sol. N on the graphite annealing time and the hardness.Numerals in circles in FIG. 8 represent the Example No., and thehardness obtained thereby is added.

The annealing time necessary for achieving HV: 120 or below could beremarkably shortened when sol. N was decreased. Generally, the hardnessof the steel materials was affected by the C content, and the influenceof ferrite hardness became remarkable when graphite was formed. Whenlarge amounts of sol. N were contained, the hardness was not loweredsufficiently at any C contents even when the annealing time was extendedup to 120 hours. It could be appreciated also that that even when thetotal N content was at the same level, the annealing time changedgreatly depending on the sol. N amount (Examples Nos. 7 and 26 andComparative Examples Nos. 57 and 60).

Minimum hardness could be lowered by lowering sol. N. The steels havingsuch a lowered amount of sol. N could be made softer than the steelshaving a large sol. N content. It could be thus appreciated that whenthe sol. N amount exceeded the limit of the present invention, theannealing time became long, though there are certain differences in theaddition elements. When annealing was cut halfway as in ComparativeExamples Nos. 65 to 67, the graphitization ratio became insufficient, sothat the hardness after annealing did not lower and cold forgeabilitybecame inferior. When the hardness was high, cuttability fell, as well.Even if a process that was economically disadvantageous was conducted byextending the annealing time, variance of the hardness was likely tooccur in radio-frequency hardening unless the graphite grain diameterwas small enough to fall within the range of the present invention.

Since the maximum grain diameter was great and diffusion of C byradio-frequency hardening was difficult in Comparative Examples Nos. 68to 71, a long heating time was necessary for obtaining a uniformhardness.

As could be seen from Comparative Examples 71 to 73, the radio-frequencyannealing heating time had to be extended so as to eliminate thevariance when the mean grain diameter was great. This became the same asoverall heating by radio-frequency heating. In consequence, control ofthe thickness of the hardened layer became difficult, and firing crackswere likely to occur.

TABLE 9 chemical components No. section C Si Mn P S sol. N total N Cr TiV Nb Zr Mo 1 Example of 0.51 1.23 0.32 0.023 0.017 0.0020 0.0025 thisinvention 2 Example of 0.54 1.87 0.82 0.023 0.017 0.0029 0.0035 thisinvention 3 Example of 0.56 1.43 1.21 0.008 0.008 0.0019 0.0026 thisinvention 4 Example of 0.52 1.17 0.45 0.012 0.030 0.0032 0.0036 thisinvention 5 Example of 0.54 1.20 0.30 0.021 0.019 0.0022 0.0042 0.25this invention 6 Example of 0.54 1.22 0.35 0.024 0.021 0.0018 0.00520.21 0.021 this invention 7 Example of 0.55 1.21 0.32 0.022 0.024 0.00220.0062 0.015 this invention 8 Example of 0.55 1.19 0.41 0.024 0.0210.0038 0.0068 0.025 0.21 this invention 9 Example of 0.52 1.16 0.500.022 0.022 0.0035 0.0055 0.031 this invention 10 Example of 0.65 1.260.35 0.017 0.012 0.0025 0.0057 0.053 this invention 11 Example of 0.511.16 0.45 0.027 0.028 0.0035 0.0045 0.12 this invention 12 Example of0.48 1.26 0.28 0.024 0.021 0.0019 0.0047 0.11 this invention 13 Exampleof 0.54 1.82 0.54 0.024 0.021 0.0029 0.0032 this invention 14 Example of0.52 1.09 0.36 0.029 0.018 0.0037 0.0055 this invention 15 Example of0.51 1.29 0.38 0.021 0.015 0.0032 0.0050 this invention 16 Example of0.53 1.25 0.36 0.029 0.018 0.0037 0.0047 this invention 17 Example of0.54 1.31 0.46 0.027 0.012 0.0017 0.0026 this invention 18 Example of0.54 1.31 0.46 0.027 0.012 0.0017 0.0036 this invention 19 Example of0.52 1.20 0.32 0.015 0.010 0.0027 0.0060 this invention

TABLE 10 graphiti- zation mean maximum chemical components ratio graingrain No. section Al B Pb Ca Te Se Bi Mg (%) diameter 10 × C^(⅓)diameter 1 Example of 0.027 79 4.2 7.99 13.2 this invention 2 Example of0.023 85 4.5 8.14 11.5 this invention 3 Example of 0.017 82 5.5 8.2410.6 this invention 4 Example 82 4.8 8.04 14.2 this invention 5 Exampleof 0.029 72 4.2 8.14 12.6 this invention 6 Example of 0.019 85 5.9 8.148.9 this invention 7 Example of 0.029 82 5.0 8.19 12.5 this invention 8Example of 0.030 76 4.6 8.19 10.3 this invention 9 Example of 0.036 734.1 8.04 14.3 this invention 10 Example of 0.021 85 3.9 8.66 14.5 thisinvention 11 Example of 0.022 86 4.8 7.99 13.5 this invention 12 Exampleof 0.034 0.0021 93 4.2 7.83 12.6 this invention 13 Example of 0.029 0.1391 4.6 8.14 14.5 this invention 14 Example of 0.027 0.0021 0.0013 86 5.08.04 18.3 this invention 15 Example of 0.021 0.0025 0.031 88 4.7 7.9912.0 this invention 16 Example of 0.023 0.0024 0.23 79 5.8 8.09 11.9this invention 17 Example of 0.027 0.30 86 5.5 8.14 13.5 this invention18 Example of 0.017 0.0060 86 5.5 8.14 13.5 this invention 19 Example of0.0045 86 5.5 8.04 13.5 this invention

TABLE 11 chemical components No. section C Si Mn P S sol. N total N CrTi V Nb Zr Mo 20 Example of 0.32 1.23 0.42 0.013 0.027 0.0021 0.0036this invention 21 Example of 0.32 1.27 0.54 0.023 0.012 0.0022 0.0040this invention 22 Example of 0.26 1.83 0.51 0.003 0.015 0.0037 0.0048this invention 23 Example of 0.32 1.17 0.45 0.020 0.025 0.0012 0.0020this invention 24 Example of 0.25 1.20 0.60 0.026 0.020 0.0032 0.00420.21 0.022 this invention 25 Example of 0.34 1.32 0.25 0.022 0.0250.0032 0.0065 0.25 0.023 this invention 26 Example of 0.35 1.21 0.360.019 0.022 0.0023 0.0065 0.035 0.25 this invention 27 Example of 0.351.19 0.81 0.027 0.023 0.0038 0.0055 0.040 this invention 28 Example of0.23 1.16 0.52 0.028 0.023 0.0041 0.0050 0.048 this invention 29 Exampleof 0.35 1.26 0.55 0.027 0.019 0.0025 0.0046 0.22 this invention 30Example of 0.31 1.26 0.75 0.028 0.025 0.0033 0.0047 0.10 this invention31 Example of 0.38 1.46 0.18 0.025 0.029 0.0015 0.0040 this invention 32Example of 0.24 1.32 0.50 0.026 0.025 0.0039 0.0038 this invention 33Example of 0.32 1.31 0.91 0.030 0.022 0.0042 0.0051 this invention 34Example of 0.32 1.20 0.34 0.021 0.026 0.0042 0.0055 this invention 35Example of 0.33 1.26 0.36 0.028 0.018 0.0037 0.0057 this invention 36Example of 0.38 1.34 0.45 0.029 0.017 0.0026 0.0036 this invention 37Example of 0.32 1.24 0.32 0.022 0.012 0.0030 0.0045 this invention

TABLE 12 graphiti- zation mean maximum chemical components ratio graingrain No. section Al B Pb Ca Te Se Bi Mg (%) diameter 10 × C^(⅓)diameter 20 Example of 0.025 81 3.5 6.84 10.1 this invention 21 Exampleof 0.022 76 3.7 6.84 9.8 this invention 22 Example 0.022 85 2.8 6.3810.6 this invention 23 Example of 88 2.4 6.84 12.3 this invention 24Example of 0.021 76 3.5 6.30 9.8 this invention 25 Example of 0.018 772.2 6.98 8.3 this invention 26 Example of 0.030 88 3.5 7.05 9.6 thisinvention 27 Example of 0.031 75 3.8 7.05 10.6 this invention 28 Exampleof 0.029 74 3.7 6.13 10.2 this invention 29 Example of 0.027 85 3.6 7.0513.2 this invention 30 Example of 0.017 89 3.5 6.77 12.5 this invention31 Example of 0.023 0.0028 91 2.8 7.24 9.8 this invention 32 Example of0.021 0.21 89 3.8 6.21 12.2 this invention 33 Example of 0.026 0.00250.0016 86 3.6 6.84 9.6 this invention 34 Example of 0.022 0.0022 0.02187 3.0 6.84 8.9 this invention 35 Example of 0.022 0.0023 0.25 95 3.66.91 7.9 this invention 36 Example of 0.025 0.29 92 3.0 7.24 9.0 thisinvention 37 Example of 0.001 0.0055 92 3.0 6.84 9.0 this invention

TABLE 13 chemical components No. section C Si Mn P S sol. N total N CrTi V Nb Zr Mo 38 Example of 0.55 0.75 0.31 0.023 0.017 0.0020 0.0032this invention 39 Example of 0.44 0.65 0.72 0.023 0.017 0.0021 0.0034this invention 40 Example of 0.36 0.50 1.01 0.008 0.008 0.0019 0.0025this invention 41 Example of 0.22 0.42 0.52 0.012 0.030 0.0042 0.0056this invention 42 Example of 0.54 0.46 0.42 0.021 0.019 0.0022 0.00380.25 this invention 43 Example of 0.54 0.21 0.51 0.024 0.021 0.00320.0052 0.007 this invention 44 Example of 0.55 0.55 0.36 0.022 0.0240.0022 0.0061 0.025 this invention 45 Example of 0.55 0.64 0.24 0.0240.021 0.0048 0.0078 0.025 0.21 this invention 46 Example of 0.52 0.430.37 0.022 0.022 0.0035 0.0049 0.031 this invention 47 Example of 0.650.51 0.38 0.017 0.012 0.0025 0.0051 0.053 this invention 48 Example of0.51 0.35 0.48 0.027 0.028 0.0035 0.0045 0.12 this invention 49 Exampleof 0.48 0.65 0.19 0.024 0.021 0.0019 0.0056 0.11 this invention 50Example of 0.54 0.78 0.62 0.024 0.021 0.0029 0.0043 this invention 51Example of 0.52 0.25 0.25 0.029 0.018 0.0037 0.0062 this invention 52Example of 0.51 0.35 0.54 0.021 0.015 0.0032 0.0055 this invention 53Example of 0.33 0.45 0.27 0.029 0.018 0.0037 0.0058 this invention 54Example of 0.44 0.32 0.29 0.027 0.012 0.0027 0.0064 0.11 this invention55 Example of 0.54 0.62 0.29 0.027 0.012 0.0021 0.0048 invention 56Example of 0.52 0.32 0.29 0.027 0.012 0.0024 0.0058 0.010 this invention

TABLE 14 graphiti- zation mean maximum chemical components ratio graingrain No. section Al B Pb Ca Te Se Bi Mg (%) diameter 10 × C^(⅓)diameter 38 Example of 0.027 79 4.7 8.19 12.5 this invention 39 Exampleof 0.023 85 4.0 7.61 13.1 this invention 40 Example of 82 3.6 7.11 10.5this invention 41 Example of 0.017 92 3.5 6.04 10.2 this invention 42Example of 0.029 72 4.9 8.14 11.3 this invention 43 Example of 0.019 855.6 8.14 11.8 this invention 44 Example of 0.029 82 5.8 8.19 14.5 thisinvention 45 Example of 0.030 76 5.5 8.19 13.0 this invention 46 Exampleof 0.036 73 4.6 8.04 12.7 this invention 47 Example of 0.021 85 4.2 8.6614.5 this invention 48 Example of 0.022 86 4.3 7.99 12.5 this invention49 Example of 0.034 0.0021 93 4.4 7.83 13.6 this invention 50 Example of0.029 0.13 91 5.2 8.14 15.2 this invention 51 Example of 0.027 0.00210.0013 86 5.4 8.04 14.4 this invention 52 Example of 0.021 0.0025 33 4.37.99 11.9 this invention 53 Example of 0.023 0.0024 46 4.1 6.91 12.0this invention 54 Example of 0.027 67 4.8 7.61 14.3 this invention 55Example of 0.027 0.0035 86 4.8 8.14 14.3 this invention 56 Example of0.0041 86 4.8 8.04 14.3 this invention

TABLE 15 chemical components No. section C Si Mn P S sol. N total N CrTi V Nb Zr Mo 57 Comparative 0.55 1.23 0.34 0.019 0.017 0.0059 0.0068Example 58 Comparative 0.49 1.19 0.40 0.021 0.020 0.0070 0.0091 Example59 Comparative 0.52 1.20 0.29 0.015 0.012 0.0068 0.0095 Example 60Comparative 0.35 1.18 0.35 0.021 0.026 0.0062 0.0075 Example 61Comparative 0.35 1.21 0.31 0.011 0.019 0.0082 0.0105 Example 62Comparative 0.53 0.75 0.41 0.029 0.027 0.0057 0.0067 Example 63Comparative 0.46 0.69 0.41 0.022 0.021 0.0061 0.0101 Example 64Comparative 0.36 0.75 0.34 0.024 0.021 0.0057 0.0069 Example 65Comparative 0.58 0.35 0.50 0.021 0.026 0.0082 0.0124 0.01 Example 66Comparative 0.46 0.38 0.34 0.023 0.019 0.0059 0.0079 Example 67Comparative 0.36 0.40 0.34 0.024 0.021 0.0057 0.0084 Example 68Comparative 0.55 1.21 0.32 0.024 0.026 0.0068 0.0083 0.11 Example 69Comparative 0.44 1.06 0.35 0.021 0.022 0.0063 0.0092 0.014 Example 70Comparative 0.47 0.71 0.50 0.029 0.021 0.0065 0.0087 Example 71Comparative 0.53 1.12 0.36 0.022 0.027 0.0035 0.0045 Example 72Comparative 0.51 1.21 0.35 0.019 0.019 0.0038 0.0058 Example 73Comparative 0.36 1.22 0.35 0.014 0.022 0.0037 0.0049 Example

TABLE 16 graphiti- zation mean maximum chemical components ratio graingrain No. section Al B Pb Ca Te Se Bi Mg (%) diameter 10 × C^(⅓)diameter 57 Comparative 0.028 65 3.4 8.19 13.8 Example 58 Comparative0.019 0.0026 58 3.2 7.88 11.7 Example 59 Comparative 0.028 52 3.4 8.1914.8 Example 60 Comparative 55 4.2 7.05 8.7 Example 61 Comparative 0.02154 4.7 7.05 12.7 Example 62 Comparative 0.028 48 4.6 8.09 10.5 Example63 Comparative 0.019 42 4.5 7.72 12.9 Example 64 Comparative 0.0210.0024 41 4.7 7.11 13.8 Example 65 Comparative 0.010 15 4.4 8.34 10.5Example 66 Comparative 0.019 18 4.5 7.72 12.5 Example 67 Comparative0.021 0.0024 16 1.5 7.11 10.0 Example 68 Comparative 0.015 85 4.3 8.1925.1 Example 69 Comparative 0.027 0.0021 64 4.6 7.61 26.9 Example 70Comparative 0.021 0.0021 79 3.6 7.78 31.0 Example 71 Comparative 0.0280.0025 78 9.1 8.09 21.6 Example 72 Comparative 0.027 0.0028 89 9.4 7.9914.8 Example 73 Comparative 0.022 0.0021 45 7.7 7.11 16.8 Example

TABLE 17 annealing annealing heating No. section machinability timehardness (HV) time hardness 1 Example of 150 8 121 3 645 this invention2 Example of 150 8 124 3 657 this invention 3 Example of 150 8 127 3 721this invention 4 Example of 150 14 126 3 581 this invention 5 Example of150 12 125 3 594 this invention 6 Example of 150 8 120 3 679 thisinvention 7 Example of 150 12 122 3 702 this invention 8 Example of 1506 128 3 712 this invention 9 Example of 150 6 124 3 680 this invention10 Example of 150 8 126 3 750 this invention 11 Example of 150 8 119 3654 this invention 12 Example of 150 16 126 3 621 this invention 13Example of 150 16 127 3 655 this invention 14 Example of 150 8 124 6 649this invention 15 Example of 150 8 125 3 635 this invention 16 Exampleof 150 8 120 3 681 this invention 17 Example of 150 8 123 3 678 thisinvention 18 Example of 150 8 123 3 678 this invention 19 Example of 1508 123 3 678 this invention 20 Example of 150 4 119 3 452 this invention21 Example of 150 4 125 3 458 this invention 22 Example of 150 6 124 3432 this invention 23 Example of 150 4 122 3 452 this invention 24Example of 150 4 128 3 401 this invention 25 Example of 150 4 124 3 459this invention 26 Example of 150 6 118 3 481 this invention 27 Exampleof 150 4 125 3 446 this invention 28 Example of 150 4 126 3 385 thisinvention

TABLE 18 annealing annealing heating No. section machinability timehardness (HV) time hardness 29 Example of 150 6 119 3 446 this invention30 Example of 150 6 120 3 450 this invention 31 Example of 150 6 118 3521 this invention 32 Example of 150 6 125 3 385 this invention 33Example of 150 6 131 3 450 this invention 34 Example of 150 6 125 3 461this invention 35 Example of 150 6 109 3 463 this invention 36 Exampleof 150 6 121 3 501 this invention 37 Example of 150 6 121 3 501 thisinvention 38 Example of 150 8 121 3 681 this invention 39 Example of 1508 124 3 592 this invention 40 Example of 150 8 127 3 450 this invention41 Example of 150 8 126 3 392 this invention 42 Example of 150 12 125 3681 this invention 43 Example of 150 8 122 3 702 this invention 44Example of 150 12 120 3 721 this invention 45 Example of 150 6 128 3 681this invention 46 Example of 150 6 124 3 677 this invention 47 Exampleof 150 8 126 3 730 this invention 48 Example of 150 8 119 3 624 thisinvention 49 Example of 150 16 126 3 623 this invention 50 Example of150 16 127 3 592 this invention 51 Example of 150 8 124 3 681 thisinvention 52 Example of 150 8 125 3 653 this invention 53 Example of 1508 120 3 693 this invention 54 Example of 150 8 123 3 672 this invention55 Example of 150 8 123 3 672 this invention 56 Example of 150 8 123 3672 this invention

TABLE 19 annealing annealing heating No. section machinability timehardness (HV) time hardness 57 Comparative 60 60 138 3 648 Example 58Comparative 70 60 141 3 589 Example 59 Comparative 70 120 135 7 631Example 60 Comparative 100 72 145 3 460 Example 61 Comparative 90 120132 3 454 Example 62 Comparative 70 120 145 3 659 Example 63 Comparative60 120 141 3 601 Example 64 Comparative 100 120 135 3 452 Example 65Comparative 50 16 152 3 720 Example 66 Comparative 50 16 141 3 601Example 67 Comparative 60 8 145 3 452 Example 68 Comparative 100 120 15215 759 Example 69 Comparative 80 120 148 12 589 Example 70 Comparative100 120 148 10 592 Example 71 Comparative 120 48 125 12 625 Example 72Comparative 120 32 126 12 752 Example 73 Comparative 120 24 126 8 453Example

INDUSTRIAL APPLICABILITY

The steel for cold forging according to the present invention hasexcellent surface hardness, excellent deformation properties andmachinability, and can be used either as-rolled or under an annealedstate for a short time. Moreover, because the steel contains C, thestrength can be remarkably improved by heat-treatment, and mechanicalcomponents can be produced easily and highly efficiently. Furthermore,the steel for cold forging according to the present invention canshorten the annealing time for softening.

What is claimed is:
 1. A structural steel for cold forging, excellent insurface layer hardness and softening properties by annealing, consistingessentially of, in terms of wt %: C: 0.1 to 1.0%, Si: 0.1 to 2.0%, Mn:0.01 to 1.50%, P: not greater than 0.100%, S: not greater than 0.500%,Sol N: being limited to not greater than 0.005%, Mg: 0.0005 to 0.02%,and the balance consisting of Fe and unavoidable impurities: wherein apearlite ratio in the steel structure (pearlite occupying area ratio inmicroscope plate/microscope plate area) is not greater than 120×(C %)(with the proviso that the ratio is not greater than 100%), and theoutermost layer hardness is at least 450×(C %)+90 in terms of theVickers hardness Hv.
 2. A structural steel for cold forging, excellentin cold formability, cuttability and radio-frequency hardenability,consisting essentially of, in terms of wt %: C: 0.1 to 1.0%, Si: 0.1 to2.0%, Mn: 0.01 to 1.50%, P: not greater than 0.100%, S: not greater than0.500%, Sol N: being limited to not greater than 0.005%, Mg: 0.0005 to0.02%, and the balance consisting of Fe and unavoidable impurities, andhaving structure, wherein: a ratio of graphite amount to the carboncontent in the steel (graphitization ratio: the amount of carbonprecipitated as graphite/the carbon content in the steel) exceeds 20%, amean grain diameter of graphite is not greater than 10×(C%)^(⅓) μm, anda maximum grain diameter is not greater than 20 μm.
 3. A structuralsteel for cold forging, excellent in surface layer hardness andsoftening properties by annealing, and/or excellent in cold formability,cuttability and radio-frequency hardenability, according to claim 1 or2, wherein the steel further contains at least one of Cr: 0.01 to 0.70%,Mo: 0.05 to 0.50%, Ti: 0.01 to 0.20%, V: 0.05 to 0.50%, Nb: 0.01 to0.10%, Zr: 0.01 to 0.30%, Al: 0.001 to 0.50%, B: 0.0001 to 0.0060%, Pb:0.01 to 0.30%, Ca: 0.0001 to 0.0020%, Te: 0.001 to 0.1000%, Se: 0.01 to0.50%, Bi: 0.01 to 0.50%.
 4. A method for producing a structural steelfor cold forging, excellent in surface layer hardness and softeningproperties by annealing, the method comprising the steps of: hot-rollinga steel consisting essentially of, in terms of wt %: C: 0.1 to 1.0%, Si:0.1 to 2.0%, Mn: 0.01 to 1.50%, P: not greater than 0.100%, S: notgreater than 0.500%, Sol N: being limited to not greater than 0.005%,Mg: 0.0005 to 0.02%, and the balance consisting of Fe and unavoidableimpurities; said hot rolling taking place in an austenite temperaturezone or in an austenite-ferrite dual phase zone so that a pearlite ratioin the steel structure (pearlite occupying area ratio in microscopeplate/microscope plate area) is not greater than 120×(C %) % (with theproviso that the ratio is not greater than 100%), and the outermostlayer hardness is at least 450×(C %)+90 in terms of the Vickers hardnessHv, cooling the hot-rolled steel immediately after the hot-rolling at acooling rate of not lower than 1° C./sec, and controlling a recuperativetemperature to 650° C. or below.
 5. A method for producing a structuralsteel for cold forging, excellent in cold formability, cuttability andradio-frequency hardenability, the method comprising the steps of:hot-rolling a steel consisting essentially of, in terms of wt %. C: 0.1to 1.0%, Si: 0.1 to 2.0%, Mn: 0.01 to 1.50%, P: not greater than 0.100%,S: not greater than 0.500%, Sol N: being limited to not greater than0.005%, Mg: 0.0005 to 0.02%, and the balance consisting of Fe andunavoidable impurities; said hot rolling taking place in an austenitetemperature zone or in an austenite-ferrite dual phase zone to obtain astructure having a ratio of graphite amount of the carbon content in thesteel (graphitization ratio: the amount of carbon precipitated asgraphite/the carbon content in the steel) exceeds 20%, a mean graindiameter of graphite is not greater than 10×(C %)^(⅓) μm, and a maximumgrain diameter is not greater than 20 μm, cooling the hot-rolled steelimmediately after the hot-rolling at a cooling rate of not lower than 1°C./sec, controlling a recuperative temperature to 650° C. or below, andgraphitization annealing the recuperated steel at a temperature in therange of 600° C. to 710° C.
 6. A method for producing a structural steelfor cold forging, excellent in surface layer hardness and softeningproperties by annealing, and/or excellent in cold formability,cuttability and ratio-frequency hardenability, according to claim 4 or5, wherein the steel further contains at least one of Cr: 0.01 to 0.70%,Mo: 0.05 to 0.50%, Ti: 0.01 to 0.20%, V: 0.05 to 0.506, Nb: 0.01 to0.10%, Zr: 0.01 to 0.30%, Al: 0.001 to 0.050%, B: 0.0001 to 0.0060%, Pb:0.01 to 0.30%, Ca: 0.0001 to 0.0020%, Te: 0.001 to 0.1000, Se: 0.01 to0.50%, Bi: 0.01 to 0.50%.